The present invention relates in general to ultrasonic testing, and in particular to a new and useful method of distinguishing between hydrogen damage in a boiler tube, and other surface conditions which give false readings that are similar to those caused by hydrogen damage.
A test for detecting hydrogen damage in fossil fired boiler tubes has been commercially utilized for several years. The test is based upon ultrasonic attenuation and has been confirmed by metallographic analysis. In some units, the test has been successfully applied to a full water wall inspection.
As a result of the large amounts of ultrasonic couplant required for a full inspection and other practical problems associated with maintaining adequate coupling, the test was modified to be performed with electromagnetic acoustic transducers (EMATs). EMATs eliminated the need for a couplant and thus allowed large areas to be scanned very rapidly. The use of EMATs also relied upon ultrasonic attenuation for the detection of damage. Both the conventional and EMAT techniques for the detection of hydrogen damage are disclosed in U.S. Pat. 4,685,344, assigned to Babcock & Wilcox, a McDermott Company.
One disadvantage of both techniques is that both techniques are sensitive to corrosion and pitting on the I.D. surface of the boiler tube. Therefore, in some instances there is no method of distinguishing between attenuation caused by actual hydrogen damage and I.D. surface conditions.
The present invention involves a supplementary technique that distinguishes between indications resulting from I.D. surface conditions and actual hydrogen damage. The inventive technique is based upon the refraction of shear waves across a chord of the boiler tube using two piezoelectric transducer wedge combinations in a pitch-catch or send-receive mode. The method of the invention also relies upon ultrasonic attenuation, however, because of the propagation path, it is relatively insensitive to the I.D. condition of the boiler tube.
The use of refracted longitudinal waves with a water column for the detection of hydrogen damage has been reported in the literature. See Sloat, Kim A., and Doug Jacks, "Inspection for Hydrogen Damage in Waterwall Tubes Using Ultrasonic Techniques," Proceedings of the EPRI Conference on Failures and Inspections of Fossil-Fired Boiler Tubes, EPRI CS-3272, Bal Harbor, Fla., April 1983. This method was tested in the original investigation leading to the present invention, and found to be impractical for field use in an actual fossil unit. The use of refracted shear and longitudinal waves as a method of introducing ultrasound into a boiler tube to measure the velocity shift resulting from multiple scattering through the microvoids of hydrogen damaged material has also been reported. See Birring, A. S., D. G. Alcazar, J. J. Hanley and S. M. Gehl, "Ultrasonic Assessment of Creep and Hydrogen Damage in Fossil Plant Components," Proceedings of the Second EPRI Fossil Plant Inspection Conference, Nov. 29-Dec. 1, 1988, San Antonio, Tex. This reference explicitly states that no ultrasonic attenuation methods are applicable for confirmation of hydrogen damage in boiler tubes. The results of feasibility tests with the present invention, however, verify the opposite. There is no known instance reported in the literature where refracted shear waves have been used for confirmation of hydrogen damage in boiler tubes.
Also, see Latimer, P. J., D. M. Stevens, and T. P. Sherlock, "A NDE Method for Hydrogen Damage Detection in Boiler Tubes, " Proceedings of the EPRI Conference on Life Extension and Assessment of Fossil Plants, Wash. D.C. Jun. 2-4, 1986, EPRI CS-5208 pp. 1061-1076; Latimer, P. J. and H. L. Whaley, "Electromagnetic Transducers for Generation and Detection of Ultrasonic Waves," Accousto-Ultrasonics, Edited by John C. Duke Jr., Plenum Publishing Corporation, 1988 (Presented at the Symposium on Acousto-Ultrasonics, Jul. 12-15, 1987, Va. Polytechnic and State University, Blacksburg, Va.), and Birring, A. S., D. G. Alcazar, J. J. Hanley, and S. Gehl, "Ultrasonic Detection of Hydrogen Damage," Materials Evaluation 47 March 1989, pp. 345.